Hashing should be flat not recursive (CPP)

## Feature Request

**Description**
It’s also not clear how things like H(a,(b,c)) are handled. In any case, we need to clearly specify these things so that they can be matched by verifiers..

**Proposed Solution**
The simplest choice is to just flatten this, but I think that it may be done recursively as H(a,H(b,c)).